Although devices for measuring a tread radius of a tire are already present, conventional devices are limited to a static measurement of a part of the tire, and it was not possible to measure the tread radius over the entire circumference of the tire (for example, see Patent Documents 1 and 2).
One of characteristics required for a tire when installed on a vehicle is the steering stability. And it has been understood that the steering stability is highly correlated with the tread radius. The tread radius is subtly varied by the tire inflation at circumferential positions. Thus, it was impossible to get a grasp of the tread radius of a tire as a whole, and the tread radius could not be fully correlated with the steering stability.
In view of these circumstances, the present inventors proposed to determine the tread radius based on data when radial distances to the tread surface of a tire rotating around the tire axis are measured over the entire circumference of the tire by the use of three laser displacement meters. And it was ascertained that, according thereto, as the data include components of RRO (Radial Run Out) of the tire, a tread radius having a higher correlation with the steering stability can be obtained.
In order to obtain a tread radius from a rotating tire, however, it is necessary to consider the influence of transverse grooves in the tread surface, and novel measurement method and measurement device are required.